PCI Express (PCI-E) technology has been widely used due to the flexible multi-connecting cabling feature. For example, a PCI-E upstream device having a 16-lane port is able to communicate with one or more downstream devices via one 16-lane cable, two 8-lane cables, four 4-lane cables, or one 8-lane cable plus two 4-lane cables. As depicted in FIG. 1(a) to 1(d), a PCI Express upstream device 10, which comprises a port having 16 PCI-Express lanes, can be flexibly connected with a PCI Express downstream device 11 having 16 lanes (FIG. 1(a)), two PCI Express downstream device 12 each having 8 lanes (FIG. 1(b)), one PCI Express downstream device 12 having 8 lanes and two PCI Express downstream device 13 each having 4 lanes (FIG. 1(c)), or four PCI Express downstream device 14 each having 4 lanes (FIG. 1(d)).
However, in view of the PCI Express upstream device, what the downstream configuration will be is unknown and required to be estimated during initialization. The downstream configuration is determined according to a plurality of links, each involving a series of state transitions.